Try seeing how it behaves in bordered windowed mode. I ran a config like yours (1080 paired with a 1440) but in bordered mode. Usually it would detect the correct resolution for the monitor it's on but occasionally (one of the monitors going into sleep mode sending the game to the other monitor) it would need a little heads up by forcing a window size change, like maximizing it or just changing windows size for example. Not sure how you do that on borderless mode.

What resolution does the game say it's using?